You have a point... sort of.
Take a look at it from this angle; If you're a reputable seller and have a considerable amount of vouches (e.g. 2,000), then it allows you to sell your products (in this case, upgrades) for a "premium price". If someone had 2,000 vouches for selling upgrades and sold their upgrades for 20m and someone else had 0 vouches and was selling for 12m, I think I would trust the person with 2000 vouches and pay the extra 8m just for the sakes of not being scammed and a fast, easy trade.
Also it depends on how much the person bought them for. If they bought a few for 14m they may be hoping to flip them and sell them for some extra cash